Make this repo be a real "NeoFS k6 extension"

[carpawell]

Is your feature request related to a problem? Please describe.

Extending k6 is a natural and expected thing, there are plenty extensions already: https://k6.io/docs/extensions/get-started/explore/. However, we do not extend k6 with NeoFS in the repo called "xk6-neofs", we test NeoFS here. So it either should be named "neofs-load", or be a golang package that can be used in xk6 util with --with flag.

Describe the solution you'd like

Keep only NeoFS client here, and redesign a little this repo to look similar to every public "xk6-*" repos in public. All the helpers/scripts/load scenarios/etc move to a separate repo called "load" or smth (can be private or public depending on our goals).

Describe alternatives you've considered

1. Keep it private (cannot be done already)
2. Rename it (or move all the code to some "load" repo; return to this issue later)
3. Keep it as is (I've already created the issue, so it is obvious that I do not like it)

Additional context

An example of what should be called "xk6-*" extension (and how it should look) according to the creators: https://github.com/grafana/xk6-loki